Coded modulation method, which takes tailbits and their coding into account

ABSTRACT

A method is presented for the coded modulation of digital data, a modulation being carried out that is coded in multiple stages. In this context, the useful bits for the coded modulation are divided into parallel signal streams, and each signal stream is fed to a specific coder to carry out the channel coding. In the respective coder, the useful bits have tail bits added to them, which, in contrast to the useful bits, are channel-coded using a variable code rate, to achieve a preestablished bit number for a specific coder, so that all signal streams that are coded in parallel contain the same number of bits.

BACKGROUND INFORMATION

[0001] The present invention is based on a method for coded modulation in accordance with species of the independent claim.

[0002] It is already known to use coded modulation, in which channel coding and modulation are optimized in common. As an equivalent to the term coded modulation, the designation multilevel coding is also familiar.

ADVANTAGES OF THE INVENTION

[0003] In contrast, the method for coded modulation according to the present invention having the features of the independent claim has the advantage that, for the so-called tail bits, a variable code rate is used, which is set in each case so that the available transmission capacity can be completely exploited. The transmission framework used specifies the maximum number of modulation symbols. This results in an optimal increase in capacity in comparison to the known methods. The tail bits are added either in the coders or in the upstream bit multiplexer.

[0004] Furthermore, the method according to the present invention can be used in a flexible manner, because different transmission modes having different numbers of modulation symbols can be used. In addition, no supplemental signaling from the transmitter to the receiver is required. Finally, the implementation of the method according to the present invention is simple, because no additional computing capacity is required.

[0005] As a result of the measures and refinements indicated in the dependent claims, advantageous improvements of the method for coded modulation indicated in the independent claim are possible.

[0006] It is especially advantageous that the variable code rate is achieved by a variable puncturation. Puncturation means that in order to achieve a higher code rate, some bits are not transmitted with the rest.

[0007] It is advantageous that the variable puncturation scheme is either stored in a table in the transmitter and in the receivers or it is calculated using a familiar calculating standard, the calculating standard being available to the transmitter and to the receivers.

[0008] In addition, it is advantageous that the fixed code rate for the useful bits and a preestablished transmission rate are signaled to the receiver, so that from them the receiver can calculate the variable code rates for the tail bits, so as to be able to synchronize itself to the transmitted data.

[0009] It is also advantageous that convolutional coding is used, which is a widely used technology for channel coding. In this context, the number of the coded bits is calculated from the number of modulation symbols multiplied by the stage level of modulation m divided by number n of stages. Therefore, the available transmission capacity can be optimally exploited.

[0010] Furthermore, it is advantageous that both a transmitter as well as a receiver have a means for carrying out the method according to the present invention.

DRAWING

[0011] Exemplary embodiments of the present invention are depicted in the drawing and are discussed in greater detail in the following description.

[0012]FIG. 1 depicts a partitioning of a 4-ASK (Amplitude Shift Keying).

[0013]FIG. 2 depicts a block diagram of the transmitter according to the present invention.

[0014]FIG. 3 depicts a block diagram of the receiver according to the present invention.

[0015]FIG. 4 depicts a flowchart of the method according to the present invention.

DESCRIPTION

[0016] The digital transmission system Digital Radio Mondiale (DRM) is currently being developed for transmission bands below 30 MHZ. The decision was made to use a multilevel coding (MLC) as the channel coding. In this context, the channel coding and modulation are optimized in common, thus also making it possible to speak of coded modulation. Channel coding adds redundancy to the data, on the basis of which it is possible to detect and correct transmission errors.

[0017] In a higher-stage modulation method having a q-nary signal constellation, the signal alphabet has exactly q values. The foundation for the MLC is formed by the partitioning of the signal alphabet into partial quantities. Each division step has assigned to it one component of the address vector of the signal space representation. In this context, each component is protected by its own code. If it is a question of a 2 m-stage signal constellation, there is a division into n stages, assuming m=n, in accordance with address vector c (=c₀, c₁, . . . c_(n-1)). Stage level m of the modulation, for example, is not necessarily equal to the number of stages, if a 64-QAM (quadrature amplitude modulation) is used.

[0018]FIG. 1 depicts the partitioning of a 4ASK (Amplitude Shift Keying). In the 4ASK, therefore, four states are coded. The coding of the data stream is accomplished using n parallel coders, code c₀ having the smallest code rate R₀, i.e., it adds the most redundancy, and it protects the position of the address vector most susceptible to error. In FIG. 1, on the topmost state flow, four states are marked using filled-in circles. Via the middle two state flows, access can be achieved to the individual codable states in a 4ASK. With regard to the stage, either c₀=0 or 1. Correspondingly, the four filled-in circles are divided into two number streams, which have filled-in and empty circles, complementing each other. In the lower four state flows, the individual states are now coded in a 4ASK, namely 00, 01, 10, and 11. In this context, state 00 has a filled-in circle at the far left, which is then followed by three empty circles. State 01 has the filled-in circle in the third position from the left. State 10 has the filled-in circle at the second position from the left, and state 11 has the filled-in circle at the far right. The other positions have empty circles, symbolizing 0.

[0019] In FIG. 2, a block diagram of the transmitter according to the present invention is depicted. In data memory 1, data are contained that are to be transmitted using the transmitter according to the present invention. However, other data sources can also be used here. These data are transmitted from data memory 1 to a source coder 2, which undertakes a source coding to reduce the quantity of the data to be transmitted.

[0020] The data that are source-coded in this manner and that have the useful bits are then transmitted to a multiplexer 3, which divides the data stream into n parallel lines. To each of these n lines, which are numbered 0 through n-1, a specific coder is connected, which channel-codes one of data streams (q₀ . . . q_(n-1)). For example, here a coder 5 in line 0 and a coder 4 in line n-1 are depicted. Present at the output of each coder are signals c₀ or c_(n-1). Coders 4 and 5 execute the channel coding using a convolutional coding and, in the process, add additional redundancy to the useful bits, so-called tail bits also being added to the useful bits, so as to place each coder 4 and 5, as convolutional coders, into a defined terminal state. A coder 4 and 5 of this type has shift registers, which are each wired in accordance with the coding. The tail bits, here depicted as logical zeros, assure that coders 4 and 5, at the end of the coding, are in a defined state and that, in the receiver, the decoder, at the end of the decoding, is also in a defined state, which is recognized by the fact that all the bits in coders 4 and 5, or the decoders, are logical zeros.

[0021] These tail bits are also acted upon by a code rate, which here, however, in accordance with the present invention, is variable. This variable code rate is set so that the available transmission capacity, which is defined by the transmission frame, is completely used. Variable here means that the code rate of the tail bits can be different for each stage. As a result of the variable number of tail bits, it is possible that the number of coded bits, i.e., the coded useful bits plus the coded tail bits, is the same for every stage of the coding scheme. In addition, the number of the coded bits then corresponds to the number of the modulation symbols multiplied by the stage level of modulation m and divided by the number of stages n. In a 4- or 8-ASK, m=n, so that the number of coded bits corresponds to the number of modulation symbols. In a 64-QAM, however, it is possible that m=6 and n=3, so that the number of coded bits corresponds to double the number of modulation symbols.

[0022] The data that are channel-coded in this manner are then assigned to signal space points in block 6, in order then to generate the specific modulation symbols.

[0023] As the component codes in individual coders 4 and 5, convolutional codes having puncturation are used. Therefore, the code rates can be adjusted to each other in order to achieve the best possible transmission performance. The punctured codes have a period that corresponds to the denominator of the code rate. For example, code 4/5 has precisely five output bits per four input bits. Therefore, the period of the output bits is 5, because no smaller number of output bits is possible, so as to maintain the code rate. In the MLC, a different code rate is used for each stage,. To assure that the number of bits at the outputs of all coders 4 and 5 is the same, this number must accordingly be variable. Of course, this only applies to the tail bits, because the code rate for the useful bits remains the same. By changing the code rate of the tail bits, it is possible to influence the number of code bits. In this context, the assumption is made of a maximum code rate for the tail bits, which is reduced, i.e., additional redundancy is added, so as to accomplish an adjustment. In this context, the reduction of the code rate of the tail bits must take place for every stage to that degree, so that output bits occur in every stage, and they correspond to the number of modulation symbols multiplied by m and divided by n. The number of tail bits can be established for every value in a specific range using the puncturation schema. Alternatively, a minimal code rate can be the point of departure, and it can be increased by adjusting the puncturation.

[0024] The following example is to clarify the usefulness. When convolutional codes having memory length 6 are used, six tail bits are required to place coder 4 or 5 in a defined terminal state. This defined terminal state is achieved for each transmission frame, in order to prevent the propagation of errors during the decoding in the receiver. The tail bits can be added either in coders 4 and 5 or in multiplexer 3. Acting as a foundation is an 8-ASK as modulation, having m=3 and an MLC having n=3 stages. If the tail bits are provided with a base code rate of ⅔, then precisely nine coded bits arise, corresponding to the six zeros (tail bits) at the coder input. If there are 200 modulation symbols in a transmission frame, then ideally 200 coded bits are produced per stage. If the minimum number of nine tail bits is removed therefrom, then 191 coded useful bits are generated as the maximum possible number per stage. If the period is considered for every stage, then the result is that for stage 0 having a code rate of ⅓, exactly 189 coded bits arise (corresponding, at the code rate, to 63 useful bits) and consequently 11 tail bits are required. For stage 1 having a code rate of ⅔, exactly 189 coded bits are present (corresponding, at the present code rate, to 126 useful bits), so that once again 11 tail bits are necessary. In stage 2, there are 190 coded bits at a code rate of ⅘ (corresponding to 152 useful bits), and ten tail bits are therefore necessary. The code rates of the tail bits of the stages are changed from the base code rate ⅔ to {fraction (6/11)} for stage 0 and stage 1, or to {fraction (6/10)} for stage 2. Thus it is achieved that all modulation symbols are allocated using coded bits. The result of this calculation is that 341 useful bits are transmitted, which amounts to an increase in capacity in comparison to conventional methods. In this context, the result is 568 coded bits, which correspond to 341 useful bits. In conventional methods using code rates ⅘ and ⅔, it would be necessary to select a number of coded bits, and therefore of useful bits, that is divisible by 3 and 5. Using the variable code rate, it is now possible to achieve an optimum value for the coded bits.

[0025] In FIG. 2, the modulation symbols coded in this manner are transmitted from function block 6 to an OFDM modulator 7, which divides the individual modulation signals into frequency carriers that are situated close to each other, and that are orthogonal with respect to each other. The OFDM signals that arise in this manner, are then mixed in an analog high-frequency part 8, amplified, and then broadcast using an antenna 9.

[0026] In FIG. 3, a block diagram of the receiver according to the present invention is depicted. An antenna 10 is connected to is an input of a high-frequency receiving part 11 for receiving the OFDM signals. High-frequency receiving part 11 converts the received signals to an intermediate frequency, and then amplifies and filters them. In addition, high-frequency receiving part 11 transmits these signals to a digital part 12, which digitalizes the received signals and carries out an OFDM demodulation. The modulation symbols obtained in this manner are then demodulated by a processor 13 and are converted into a data stream, which is transformed by processor 13 into analog signals, which are then amplified by audio amplifier 14, so as finally to be reproduced using loudspeaker 15. Alternatively, it is also possible here to receive multimedia data which are then reproduced optically. In addition, the transmitter signals to the receivers what the fixed code rate is for the channel coding of the useful bits and what the transmission rate is. In this way, it is then possible for the receivers to determine the specific variable code rate for the tail bits.

[0027] In FIG. 4, the method according to the present invention is depicted as a flowchart. In method step 16, the data are made available by data memory 1 and are subjected to a source coding by source coder 2. In method step 17, the data stream that arises in this manner is divided by bit multiplexer 3 into data streams that run in parallel. In method step 18, the individual coders carry out the channel coding, the useful bits being coded using a fixed code rate, and the tail bits using a variable code rate, which is a function of whether the number of output bits of individual coders 4 and 5 agrees with the number of modulation symbols. This is achieved using the so-called puncturation scheme of the tail bits of each stage. In method step 20, in function block 6, signal space points are assigned to the channel-coded data that arise in this manner so as to generate the modulation symbols. In method step 21, the modulation symbols are subjected to an OFDM modulation, and in method step 22 the amplification, or transmission, of the OFDM signals takes place. In this context, the fixed code rate for the useful bits and the transmission rate are also transmitted by the transmitter as signaling to the receivers, so that the receivers are able to calculate the variable code rates on the basis of the received data. 

What is claimed is:
 1. A method for the coded modulation of digital data, the digital data having useful bits, the coded modulation being carried out in a multi-stage manner, wherein the useful bits for the coded modulation are divided into parallel signal streams; each signal stream is subjected by a specific coder (4, 5) to a channel coding, the useful bits being channel-coded using at least one fixed code rate; the useful bits have tail bits added to them; the tail bits are channel-coded in the respective coder (4, 5) using a variable code rate to achieve a preestablished bit number for the respective coder (4, 5), so that all signal streams that are coded in parallel contain the same number of bits; and the channel-coded data are assigned to signal space points so as to generate modulation symbols.
 2. The method as recited in claim 1, wherein the variable code rate of the tail bits is achieved using variable puncturation.
 3. The method as recited in claim 2, wherein all possible puncturation schemas are made available to a transmitter and receivers.
 4. The method as recited in claim 2, wherein all possible puncturation schemas of the tail bits are calculated, the calculation methods being available to a transmitter as well as receivers.
 5. The method as recited in claim 3 or 4, wherein the fixed code rate and a transmission rate are signaled to the receivers.
 6. The method as recited in one of the preceding claims, wherein a convolutional coding is used as the channel coding.
 7. The method as recited in one of the preceding claims, wherein the number of coded bits is generated by the number of modulation symbols multiplied by the stage level m of the modulation and divided by the number of stages.
 8. A transmitter for carrying out the method as recited in one of claims 1 through 7, wherein the transmitter has a multiplexer (3) for dividing the useful bits into the parallel signal streams of a number of coders (4, 5) and a means (6) for assigning the coded data to the signal space points, the number of coders (4, 5) corresponding to the number of stages of the modulation.
 9. A receiver for carrying out the method according to claims 3, 4, 5, 6, or 7, wherein the receiver has a means for evaluating the signaling (12, 13) for the demodulating of the modulation symbols and for the channel coding. 